Breast cancer is the second most common cancer in the United States. Based on the most recent data, one in eight women will be diagnosed with breast cancer at some point during her lifetime. Chemotherapy (chemo) and radiation treatment (RT) are standard treatments for this disease. Unfortunately, a large subset of breast cancer patients will be resistant to treatment. Therefore, understanding the mechanisms that lead to treatment resistance is crucial to improving breast cancer survival. Previously, a 49-gene signature primarily comprised of interferon-stimulated genes (ISGs) was discovered to predict breast cancer patients that are resistant to chemo/RT, and termed the interferon-related DNA damage signature (IRDS). Recently, it was determined that the tumor microenvironment regulates the IRDS. A subgroup of breast cancers, enriched in the basal subtype, signal to stromal fibroblasts (stroma) to upregulate the vesicular trafficking protein RAB27B. The induction of RAB27B results in increased secretion of exosomes by stroma and their subsequent transfer to breast cancer cells. Uptake of exosomes instigates high-level expression of the IRDS through the viral pattern recognition receptor (PRR) RIG-I and a downstream anti-viral transcription factor STAT1. STAT1 mediates the protective effects of stroma by this exosome-driven anti-viral pathway. Interestingly, tumor-stroma conditioned media enriched in exosomes can horizontally transfer the protective effects of stroma when added to breast cancer cells that normally are unable to coerce stroma to either augment exosome secretion or confer protection. Exosomes contain DNA, RNA, and protein~ however, the functional cargo is largely unknown. RIG-I is known to be activated by viral RNA to induce ISGs in immune and non-immune cells. Moreover, preliminary data suggests that exosomal RNA is enriched in non-coding RNA that are sufficient to induce the IRDS in a RIG-I dependent manner. In total, these results suggest that in breast cancer patients with IRDS(+) tumors, breast cancer cells may promote stroma to secrete RNA containing exosomes that subsequently drive an anti-viral pathway that influences resistance to chemo/RT. Therefore, this proposal aims to test the hypothesis that stroma transfers non-coding RNA via exosomes to breast cancer cells that then bind and activate RIG-I to induce ISGs known to control chemotherapy and radiation resistance.